1. Field of the Invention
This invention relates to a photographic system used for printing an original image on a photosensitive material such as enlarger, process camera, copying camera, etc. and more particularly to a method of and apparatus for detecting the initial positions of an easel and a lens wherein focusing operations are performed at two or three different positions to determine the initial positions of the easel and the lens.
2. Description of the Prior Art
Conventional photographic apparatus such as enlargers are designed so that an easel holding a photosensitive material and a lens can move vertically along a stay. When the easel is set in a desired position, the lens travels automatically to a position determined by the lens formula to focus an image on the photosensitive material in the easel. In one type of such focusing systems, a cam mechanism interlocked with the easel is provided to move the lens to a required position according to the position of the easel. In another type of such systems, a microcomputer is used to calculate the required position of the lens, and the lens is moved to the position by use of a servomotor by measuring the lens positions with a potentiometer.
With such conventional systems, an extreme caution must be paid not only to the accuracy of the cam mechanism and the potentiometer but also to be mounting positions thereof. This is because the lens and the easel are moved while the current positions thereof are measured with the cam mechanism and the potentiometer. In addition, the easel is considerably heavy and the stay for guiding the easel may be deformed during transportation due to the weight of the easel. In this case, extremely troublesome adjustments are required for the cam mechanism and the potentiometer.
To solve such a problem, a pulse motor may be used as a drive source to travel the easel and the lens, and the current positions of the easel and the lens may be detected by integrating the number of pulses that have been input to the pulse motor. However, when using a system in which a pulse motor is used and the positions of the easel and the lens are detected based on the number of pulses input to the pulse motor, it is necessary to specify the initial positions of the easel and the lens. For this purpose, indicators or the like may be mounted on the stay to indicate the initial positions of the easel and the lens. However, this method using indicators or the like involves the same disadvantages as those encountered in the conventional enlargers described above.
Moreover, with the method wherein the initial positions of the lens and of the easel are specified by a mechanical means, the position of the principal point of the lens must be corrected when the lens has been replaced with a new one. This is because the distance between the lens mounting plane and the position of the principal point of the lens alters due to the lens replacement.